A turbocharger is one of known superchargers that are used as means for enhancing power and performance of an automobile engine. The turbocharger is an apparatus that uses the energy of the exhaust gas of the engine to drive a turbine to rotate a compressor, thereby achieving a supercharged state in the engine that cannot be achieved by natural aspiration. However, when the engine runs at low speed, the exhaust flow rate is also low, and therefore, the turbine rotor does not satisfactorily rotate. Therefore, in a high-speed engine, the turbocharger has a disadvantage that the turbocharger takes a long time to efficiently drive the turbine and requires a certain time, a so-called turbo lag, to speed up the engine after that, for example. In addition, in a low-speed, diesel engine, the turbocharger has a disadvantage that it is difficult to provide the turbo effect.
To overcome the disadvantages, a VGS type turbocharger (VGS unit) that can efficiently operate even at low rotational speeds has been developed. When the engine runs at low speed, the VGS type turbocharger appropriately constricts the reduced flow of the exhaust gas with adjustable vanes (wings) to increase the velocity of the exhaust gas, thereby increasing the work of the turbine rotor, so that the VGS type turbocharger can achieve high power even when the engine runs at low speed. Therefore, the VGS unit additionally requires an adjustment mechanism for the adjustable vanes or the like, and thus the peripheral components are also inevitably complicated in configuration or the like compared with the conventional ones.
In view of such circumstances, the applicant also has been committed to research and development of the VGS type turbocharger and applied for many patents (see Patent Literature 1 to 8, for example).
The exhaust guide assembly of the VGS type turbocharger has an adjustment mechanism that includes a drive ring for uniformly opening and closing a plurality of adjustable vanes arranged at regular intervals in a circular configuration. The adjustment mechanism is configured so that the drive ring is driven to rotate by an external actuator, a lever plate transfers the rotational movement of the drive ring to the plurality of adjustable vanes to concurrently and uniformly open or close (rotate) the adjustable vanes. The adjustable vanes and a mechanism for moving the adjustable vanes will be described in more detail below. As schematically shown in FIG. 1, a number of adjustable vanes 1 are arranged in a circular configuration. A lever plate 5 is fixed at one end thereof to one end of the shaft part of each adjustable vane 1 like a crank. The lever plate 5 is engaged with an actuating drive ring 31 at the other end like a link. Rotation of the drive ring 31 causes rotation of the lever plate 5 about the shaft of the adjustable vane 1, thereby changing the angle of the adjustable vane 1.
There are various types of conventional lever plates. FIG. 8 shows an example of such conventional lever plates. A conventional lever plate 105 has an elongated plate main body 151, a fitting hole 152 into which the shaft part of the adjustable vane is fitted formed in the plate main body 151 at one end thereof, and an engaging protrusion 153 to be engaged with the drive ring formed on the plate main body 151 at the other end thereof. When the lever plate is molded by plastic working, the engaging protrusion is formed by raising a part of the plate main body. Therefore, the material of the lever plate prepared has a thickness t0 enough to accommodate for the working. As a result, there are technical difficulties in reducing the weight of the lever plate 105 and in piercing the thick member to form the fitting hole 152 for fixing the shaft part of the adjustable vane, for example. More specifically, it is difficult to choose an appropriate piercing punch for forming the hole. In addition, even if the piercing can be successfully done, the piercing punch wears out quickly because of the high load of the working, and as a result, the cost of maintenance or the like of the entire manufacturing apparatus cannot be reduced satisfactorily. In addition, of course, if maintenance is inadequate, the precision of the manufactured product is inevitably lowered.
In addition, the engaging protrusion, which is the most important functional part, is restricted by the thickness of the material, and therefore, the height of the engaging protrusion required to ensure the reliable engagement cannot be arbitrarily set.